The output voltage waveforms of switching-type inverters ordinarily contain many high order harmonics that are undesirable. A well-known technique of the prior art for reducing some of the harmonics is to cancel them by algebraically or vectorially adding together waveforms having out-of-phase harmonics of equal frequency and amplitude. One technique of the prior art is to employ two 3-phase inverters to drive a single load, with the output voltages of one of the inverters phase shifted with respect to the output voltages of the other. In load sharing schemes of this type, paralleling reactors are sometimes employed, with each extremity of each paralleling reactor connected to one corresponding phase terminal of each inverter and a center-tap of the paralleling reactor connected to the load. When the phase shift between the inverters is 30.degree. many of the significant harmonics at the load are reduced 50% by this scheme.
Other circuit connections used in the prior art for reducing harmonic components involve adding transformed voltage components in series to produce each phase of the load voltage. Where the inverters are pulse-width modulated, so that the output phase voltage waveform of each inverter has more than the usual number of steps in each cycle, some harmonics have larger amplitudes, making some form of harmonic reduction very desirable. Moreover, some pulse-width modulated waveforms could have relatively large circulation of certain harmonic currents between the two paralleled inverters. Isolation transformers have been used in the prior art to prevent such circulation.